All lens specifications include how many lens elements in how many groups the lens has. Two similar lenses from two different manufacturers: One has 13 elements in 10 groups and one has 14 elements in 12 groups. What does that mean? How does that information help one evaluate the two lenses?
Unless you're a serious optics science junkie, I wouldn't be too concerned about the optical formula for a lens, or using these formulas as a guide for quality. The true test of any lens is whether or not it meets your needs and expectations.
-PB
Posted by Dan Borzynski on March 5, 2007 at 12:19 AM
Preston is basically right - ignore that information and concentrate on the results.
To your question: Individual pieces of glass are sometimes situated right next to each other. If you take apart a lens with 3 groups and 4 elements (pieces of glass), what you would find is 2 of the glass pieces will be sitting by themselves (that is two of the groups) and the last group will have two glass pieces next to each other, or maybe even cemented to each other.
As far as I am aware of, the only place where is information is actually useful to the average photographer is if you buy closeup diopters. A one element diopter is a pretty basic design, and generally not real good. The diopters from Canon and Nikon (and nowdays, other manufacturers as well) you'll see have a couple of elements (pieces of glass) to correct for slight defects introduced by a single element.
Your answer doesn't surprise me, Preston, but still I wonder why this information is always included in every lens specification if it's too esoteric to be of any use to most users.
Isn't there some rule of thumb? Which is better? More or fewer elements? More or fewer groups? If it's too complicated to undestand, why is this specification almost universally stated? Surely there must be some useful information there.
By the way, I am a science junkie. While uneducated in optics, as an engineer I'm reasonbly fluent in math and physics. I drives me nuts to see that information and not have a clue. I was hoping that there'd actually be a serious optics guru here who could shed some light or point me in the right direction.
Posted by Richard Wong on March 5, 2007 at 1:52 AM
For marketing purposes would be my uneducated guess. I know nothing about lenses either other than if the people on fredmiranda.com like it or not.
I am a science junkie, too. I teach fire chemistry and physics. So, I understand your frustration. I am certainly not an optics guru.
Since I shoot 4x5 (mostly) the lenses are very uncomplicated in their opitcal formulae. They do not have to have any moving groups, since they have no focus mechanism or zoom capability. The lens elements and groups serve only to gather the light, eliminate abberations and project an image onto the groundglass.
Lenses that provide zoom or internal focusing are more complex, of course. Is there a rule of thumb? Which formulation is better? I don't know. It seems to me that a lens that is well designed optically and mechanically will do its job. I would assume that as the number of pieces of glass and air/glass interfaces increases, the potential for flare and other abberations will increase, too.
I always considered the optical specs given in the ads to be mostly a marketing gimmick; but maybe not. It would be great if an optics guru could shed some light on this subject for us.
-PB
Posted by Mark Stahlke on March 5, 2007 at 1:58 AM
Each air to glass interface is an opportunity for internal reflections which reduce contrast. In theory, a lens with fewer air to glass interfaces, that is fewer groups (discounting the case of air spaced elements), should be contrastier. In practice, modern multi-coating makes this a moot point.
In the case of early zoom lenses, built before modern multi-coating was developed, poor contrast was a big problem. The number of groups and elements might have been usefull for comparing different lenses back then. One could argue that multi-coating had to be invented to make zoom lenses practical.
In LF lenses, which are simple designs compared to zooms, the difference between older single-coated and newer multi-coated lenses is minimal.
In general the optical construction is of small value to the photographer. In the case of most 35mm camera objectives, you can get an idea of possible aberrations if you know where to look. For example a symmetric lens, such as a gauss design as used in most 50mm FL lenses, will have very low distortion. A triplet design, like the Elmars and Tessars, are more limited in the wider angles with coma and light fall-off being a problem. Reverse telephoto (retrofocus) designs are susceptable to barrel distortion and should be checked if you purchase one. Conversely telephotos often have pincushion distortion. Both lens designs usually have some correcting optics which can lead to a lesser "moustache" distortion.
If you have any further interest in lens construction, I would recommend a couple small books on the subject: "Lenses in Photography" by Rudolf Kingslake and "Photographic Optics" by Allen Greenleaf.
Discussion
7 repliesUnless you're a serious optics science junkie, I wouldn't be too concerned about the optical formula for a lens, or using these formulas as a guide for quality. The true test of any lens is whether or not it meets your needs and expectations.
-PB
To your question: Individual pieces of glass are sometimes situated right next to each other. If you take apart a lens with 3 groups and 4 elements (pieces of glass), what you would find is 2 of the glass pieces will be sitting by themselves (that is two of the groups) and the last group will have two glass pieces next to each other, or maybe even cemented to each other.
As far as I am aware of, the only place where is information is actually useful to the average photographer is if you buy closeup diopters. A one element diopter is a pretty basic design, and generally not real good. The diopters from Canon and Nikon (and nowdays, other manufacturers as well) you'll see have a couple of elements (pieces of glass) to correct for slight defects introduced by a single element.
Isn't there some rule of thumb? Which is better? More or fewer elements? More or fewer groups? If it's too complicated to undestand, why is this specification almost universally stated? Surely there must be some useful information there.
By the way, I am a science junkie. While uneducated in optics, as an engineer I'm reasonbly fluent in math and physics. I drives me nuts to see that information and not have a clue. I was hoping that there'd actually be a serious optics guru here who could shed some light or point me in the right direction.
I am a science junkie, too. I teach fire chemistry and physics. So, I understand your frustration. I am certainly not an optics guru.
Since I shoot 4x5 (mostly) the lenses are very uncomplicated in their opitcal formulae. They do not have to have any moving groups, since they have no focus mechanism or zoom capability. The lens elements and groups serve only to gather the light, eliminate abberations and project an image onto the groundglass.
Lenses that provide zoom or internal focusing are more complex, of course. Is there a rule of thumb? Which formulation is better? I don't know. It seems to me that a lens that is well designed optically and mechanically will do its job. I would assume that as the number of pieces of glass and air/glass interfaces increases, the potential for flare and other abberations will increase, too.
I always considered the optical specs given in the ads to be mostly a marketing gimmick; but maybe not. It would be great if an optics guru could shed some light on this subject for us.
-PB
In the case of early zoom lenses, built before modern multi-coating was developed, poor contrast was a big problem. The number of groups and elements might have been usefull for comparing different lenses back then. One could argue that multi-coating had to be invented to make zoom lenses practical.
In LF lenses, which are simple designs compared to zooms, the difference between older single-coated and newer multi-coated lenses is minimal.
Cheers,
If you have any further interest in lens construction, I would recommend a couple small books on the subject: "Lenses in Photography" by Rudolf Kingslake and "Photographic Optics" by Allen Greenleaf.